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id="article-container"><h1 id="第10章-创建和管理表"><a href="#第10章-创建和管理表" class="headerlink" title="第10章_创建和管理表"></a>第10章_创建和管理表</h1><h2 id="1-基础知识"><a href="#1-基础知识" class="headerlink" title="1. 基础知识"></a>1. 基础知识</h2><h3 id="1-1-一条数据存储的过程"><a href="#1-1-一条数据存储的过程" class="headerlink" title="1.1 一条数据存储的过程"></a>1.1 一条数据存储的过程</h3><p><code>存储数据是处理数据的第一步</code>。只有正确地把数据存储起来，我们才能进行有效的处理和分析。否则，只能是一团乱麻，无从下手。</p>
<p>那么，怎样才能把用户各种经营相关的、纷繁复杂的数据，有序、高效地存储起来呢？ 在 MySQL 中，一个完整的数据存储过程总共有 4 步，分别是创建数据库、确认字段、创建数据表、插入数据。</p>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/image-20211007155810920.webp" alt="image-20211007155810920"></p>
<p>我们要先创建一个数据库，而不是直接创建数据表呢？ </p>
<p>因为从系统架构的层次上看，MySQL 数据库系统从大到小依次是<code>数据库服务器</code>、<code>数据库</code>、<code>数据表</code>、数据表的<code>行与列</code>。 </p>
<p>MySQL 数据库服务器之前已经安装。所以，我们就从创建数据库开始。</p>
<h3 id="1-2-标识符命名规则"><a href="#1-2-标识符命名规则" class="headerlink" title="1.2 标识符命名规则"></a>1.2 标识符命名规则</h3><ul>
<li>数据库名、表名不得超过30个字符，变量名限制为29个</li>
<li>必须只能包含 A–Z, a–z, 0–9, _共63个字符</li>
<li>数据库名、表名、字段名等对象名中间不要包含空格</li>
<li>同一个MySQL软件中，数据库不能同名；同一个库中，表不能重名；同一个表中，字段不能重名</li>
<li>必须保证你的字段没有和保留字、数据库系统或常用方法冲突。如果坚持使用，请在SQL语句中使用`（着重号）引起来</li>
<li>保持字段名和类型的一致性：在命名字段并为其指定数据类型的时候一定要保证一致性，假如数据类型在一个表里是整数，那在另一个表里可就别变成字符型了</li>
</ul>
<h3 id="1-3-MySQL中的数据类型"><a href="#1-3-MySQL中的数据类型" class="headerlink" title="1.3 MySQL中的数据类型"></a>1.3 MySQL中的数据类型</h3><div class="table-container">
<table>
<thead>
<tr>
<th>类型</th>
<th>类型举例</th>
</tr>
</thead>
<tbody>
<tr>
<td>整数类型</td>
<td>TINYINT、SMALLINT、MEDIUMINT、<strong>INT(或INTEGER)</strong>、BIGINT</td>
</tr>
<tr>
<td>浮点类型</td>
<td>FLOAT、DOUBLE</td>
</tr>
<tr>
<td>定点数类型</td>
<td><strong>DECIMAL</strong></td>
</tr>
<tr>
<td>位类型</td>
<td>BIT</td>
</tr>
<tr>
<td>日期时间类型</td>
<td>YEAR、TIME、<strong>DATE</strong>、DATETIME、TIMESTAMP</td>
</tr>
<tr>
<td>文本字符串类型</td>
<td>CHAR、<strong>VARCHAR</strong>、TINYTEXT、TEXT、MEDIUMTEXT、LONGTEXT</td>
</tr>
<tr>
<td>枚举类型</td>
<td>ENUM</td>
</tr>
<tr>
<td>集合类型</td>
<td>SET</td>
</tr>
<tr>
<td>二进制字符串类型</td>
<td>BINARY、VARBINARY、TINYBLOB、BLOB、MEDIUMBLOB、LONGBLOB</td>
</tr>
<tr>
<td>JSON类型</td>
<td>JSON对象、JSON数组</td>
</tr>
<tr>
<td>空间数据类型</td>
<td>单值：GEOMETRY、POINT、LINESTRING、POLYGON；<br/>集合：MULTIPOINT、MULTILINESTRING、MULTIPOLYGON、GEOMETRYCOLLECTION</td>
</tr>
</tbody>
</table>
</div>
<p>其中，常用的几类类型介绍如下：</p>
<div class="table-container">
<table>
<thead>
<tr>
<th>数据类型</th>
<th>描述</th>
</tr>
</thead>
<tbody>
<tr>
<td>INT</td>
<td>从-2^31到2^31-1的整型数据。存储大小为 4个字节</td>
</tr>
<tr>
<td>CHAR(size)</td>
<td>定长字符数据。若未指定，默认为1个字符，最大长度255</td>
</tr>
<tr>
<td>VARCHAR(size)</td>
<td>可变长字符数据，根据字符串实际长度保存，<strong>必须指定长度</strong></td>
</tr>
<tr>
<td>FLOAT(M,D)</td>
<td>单精度，占用4个字节，M=整数位+小数位，D=小数位。 D&lt;=M&lt;=255,0&lt;=D&lt;=30，默认M+D&lt;=6</td>
</tr>
<tr>
<td>DOUBLE(M,D)</td>
<td>双精度，占用8个字节，D&lt;=M&lt;=255,0&lt;=D&lt;=30，默认M+D&lt;=15</td>
</tr>
<tr>
<td>DECIMAL(M,D)</td>
<td>高精度小数，占用M+2个字节，D&lt;=M&lt;=65，0&lt;=D&lt;=30，最大取值范围与DOUBLE相同。</td>
</tr>
<tr>
<td>DATE</td>
<td>日期型数据，格式’YYYY-MM-DD’</td>
</tr>
<tr>
<td>BLOB</td>
<td>二进制形式的长文本数据，最大可达4G</td>
</tr>
<tr>
<td>TEXT</td>
<td>长文本数据，最大可达4G</td>
</tr>
</tbody>
</table>
</div>
<h2 id="2-创建和管理数据库"><a href="#2-创建和管理数据库" class="headerlink" title="2. 创建和管理数据库"></a>2. 创建和管理数据库</h2><h3 id="2-1-创建数据库"><a href="#2-1-创建数据库" class="headerlink" title="2.1 创建数据库"></a>2.1 创建数据库</h3><ul>
<li>方式1：创建数据库</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">CREATE DATABASE 数据库名; </span><br></pre></td></tr></table></figure>
<ul>
<li>方式2：创建数据库并指定字符集</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">CREATE DATABASE 数据库名 CHARACTER SET 字符集;</span><br></pre></td></tr></table></figure>
<ul>
<li>方式3：判断数据库是否已经存在，不存在则创建数据库（<code>推荐</code>）</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">CREATE DATABASE IF NOT EXISTS 数据库名; </span><br></pre></td></tr></table></figure>
<p>如果MySQL中已经存在相关的数据库，则忽略创建语句，不再创建数据库。</p>
<blockquote>
<p>注意：DATABASE 不能改名。一些可视化工具可以改名，它是建新库，把所有表复制到新库，再删旧库完成的。</p>
</blockquote>
<h3 id="2-2-使用数据库"><a href="#2-2-使用数据库" class="headerlink" title="2.2 使用数据库"></a>2.2 使用数据库</h3><ul>
<li>查看当前所有的数据库</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">SHOW DATABASES; #有一个S，代表多个数据库</span><br></pre></td></tr></table></figure>
<ul>
<li>查看当前正在使用的数据库</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">SELECT DATABASE();  #使用的一个 mysql 中的全局函数</span><br></pre></td></tr></table></figure>
<ul>
<li>查看指定库下所有的表</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">SHOW TABLES FROM 数据库名;</span><br></pre></td></tr></table></figure>
<ul>
<li>查看数据库的创建信息</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">SHOW CREATE DATABASE 数据库名;</span><br><span class="line">或者：</span><br><span class="line">SHOW CREATE DATABASE 数据库名\G</span><br></pre></td></tr></table></figure>
<ul>
<li>使用/切换数据库</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">USE 数据库名;</span><br></pre></td></tr></table></figure>
<blockquote>
<p>注意：要操作表格和数据之前必须先说明是对哪个数据库进行操作，否则就要对所有对象加上“数据库名.”。</p>
</blockquote>
<h3 id="2-3-修改数据库"><a href="#2-3-修改数据库" class="headerlink" title="2.3 修改数据库"></a>2.3 修改数据库</h3><ul>
<li>更改数据库字符集</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">ALTER DATABASE 数据库名 CHARACTER SET 字符集;  #比如：gbk、utf8等</span><br></pre></td></tr></table></figure>
<h3 id="2-4-删除数据库"><a href="#2-4-删除数据库" class="headerlink" title="2.4 删除数据库"></a>2.4 删除数据库</h3><ul>
<li>方式1：删除指定的数据库</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">DROP DATABASE 数据库名;</span><br></pre></td></tr></table></figure>
<ul>
<li>方式2：删除指定的数据库（<code>推荐</code>）</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">DROP DATABASE IF EXISTS 数据库名;</span><br></pre></td></tr></table></figure>
<h2 id="3-创建表"><a href="#3-创建表" class="headerlink" title="3. 创建表"></a>3. 创建表</h2><h3 id="3-1-创建方式1"><a href="#3-1-创建方式1" class="headerlink" title="3.1 创建方式1"></a>3.1 创建方式1</h3><ul>
<li><strong>必须具备：</strong><ul>
<li>CREATE TABLE权限</li>
<li>存储空间</li>
</ul>
</li>
<li><strong>语法格式：</strong></li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">CREATE TABLE [IF NOT EXISTS] 表名(</span><br><span class="line">	字段1, 数据类型 [约束条件] [默认值],</span><br><span class="line">	字段2, 数据类型 [约束条件] [默认值],</span><br><span class="line">	字段3, 数据类型 [约束条件] [默认值],</span><br><span class="line">	……</span><br><span class="line">	[表约束条件]</span><br><span class="line">);</span><br></pre></td></tr></table></figure>
<blockquote>
<p>加上了IF NOT EXISTS关键字，则表示：如果当前数据库中不存在要创建的数据表，则创建数据表；如果当前数据库中已经存在要创建的数据表，则忽略建表语句，不再创建数据表。</p>
</blockquote>
<ul>
<li><strong>必须指定：</strong><ul>
<li>表名</li>
<li>列名(或字段名)，数据类型，<strong>长度</strong></li>
</ul>
</li>
<li><strong>可选指定：</strong><ul>
<li>约束条件</li>
<li>默认值</li>
</ul>
</li>
<li>创建表举例1：</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line">-- 创建表</span><br><span class="line">CREATE TABLE emp (</span><br><span class="line">  -- int类型</span><br><span class="line">  emp_id INT,</span><br><span class="line">  -- 最多保存20个中英文字符</span><br><span class="line">  emp_name VARCHAR(20),</span><br><span class="line">  -- 总位数不超过15位</span><br><span class="line">  salary DOUBLE,</span><br><span class="line">  -- 日期类型</span><br><span class="line">  birthday DATE</span><br><span class="line">);</span><br></pre></td></tr></table></figure>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">DESC emp;</span><br></pre></td></tr></table></figure>
<p> <img src= "" data-lazy-src="https://www.naste.top:9000/webp/image-20211016160557995.webp" alt="image-20211016160557995"></p>
<p>MySQL在执行建表语句时，将id字段的类型设置为int(11)，这里的11实际上是int类型指定的显示宽度，默认的显示宽度为11。也可以在创建数据表的时候指定数据的显示宽度。</p>
<ul>
<li>创建表举例2：</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">CREATE TABLE dept(</span><br><span class="line">    -- int类型，自增</span><br><span class="line">	deptno INT(2) AUTO_INCREMENT,</span><br><span class="line">	dname VARCHAR(14),</span><br><span class="line">	loc VARCHAR(13),</span><br><span class="line">    -- 主键</span><br><span class="line">    PRIMARY KEY (deptno)</span><br><span class="line">);</span><br></pre></td></tr></table></figure>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">DESCRIBE dept;</span><br></pre></td></tr></table></figure>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/image-20211016160643445.webp" alt="image-20211016160643445"></p>
<blockquote>
<p>在MySQL 8.x版本中，不再推荐为INT类型指定显示长度，并在未来的版本中可能去掉这样的语法。</p>
</blockquote>
<h3 id="3-2-创建方式2"><a href="#3-2-创建方式2" class="headerlink" title="3.2 创建方式2"></a>3.2 创建方式2</h3><ul>
<li><p>使用 AS subquery 选项，<strong>将创建表和插入数据结合起来</strong></p>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554997882872.webp" alt="1554997882872"></p>
</li>
<li><p>指定的列和子查询中的列要一一对应</p>
</li>
<li><p>通过列名和默认值定义列</p>
</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">CREATE TABLE emp1 AS SELECT * FROM employees;</span><br><span class="line"></span><br><span class="line">CREATE TABLE emp2 AS SELECT * FROM employees WHERE 1=2; -- 创建的emp2是空表</span><br></pre></td></tr></table></figure>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">CREATE TABLE dept80</span><br><span class="line">AS </span><br><span class="line">SELECT  employee_id, last_name, salary*12 ANNSAL, hire_date</span><br><span class="line">FROM    employees</span><br><span class="line">WHERE   department_id = 80;</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">DESCRIBE dept80;</span><br></pre></td></tr></table></figure>
<p> <img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554997998148.webp" alt="1554997998148"></p>
<p> <img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554998004494.webp" alt="1554998004494"></p>
<h3 id="3-3-查看数据表结构"><a href="#3-3-查看数据表结构" class="headerlink" title="3.3 查看数据表结构"></a>3.3 查看数据表结构</h3><p>在MySQL中创建好数据表之后，可以查看数据表的结构。MySQL支持使用<code>DESCRIBE/DESC</code>语句查看数据表结构，也支持使用<code>SHOW CREATE TABLE</code>语句查看数据表结构。</p>
<p>语法格式如下：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">SHOW CREATE TABLE 表名\G</span><br></pre></td></tr></table></figure>
<p>使用SHOW CREATE TABLE语句不仅可以查看表创建时的详细语句，还可以查看存储引擎和字符编码。</p>
<h2 id="4-修改表"><a href="#4-修改表" class="headerlink" title="4. 修改表"></a>4. 修改表</h2><p>修改表指的是修改数据库中已经存在的数据表的结构。</p>
<p><strong>使用 ALTER TABLE 语句可以实现：</strong></p>
<ul>
<li><p>向已有的表中添加列</p>
</li>
<li><p>修改现有表中的列</p>
</li>
<li><p>删除现有表中的列</p>
</li>
<li><p>重命名现有表中的列</p>
</li>
</ul>
<h3 id="4-1-追加一个列"><a href="#4-1-追加一个列" class="headerlink" title="4.1 追加一个列"></a>4.1 追加一个列</h3><p>语法格式如下：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">ALTER TABLE 表名 ADD 【COLUMN】 字段名 字段类型 【FIRST|AFTER 字段名】;</span><br></pre></td></tr></table></figure>
<p>举例：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">ALTER TABLE dept80 </span><br><span class="line">ADD job_id varchar(15);</span><br></pre></td></tr></table></figure>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554998139815.webp" alt="1554998139815"></p>
<h3 id="4-2-修改一个列"><a href="#4-2-修改一个列" class="headerlink" title="4.2 修改一个列"></a>4.2 修改一个列</h3><ul>
<li><p>可以修改列的数据类型，长度、默认值和位置</p>
</li>
<li><p>修改字段数据类型、长度、默认值、位置的语法格式如下：</p>
</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">ALTER TABLE 表名 MODIFY 【COLUMN】 字段名1 字段类型 【DEFAULT 默认值】【FIRST|AFTER 字段名2】;</span><br></pre></td></tr></table></figure>
<ul>
<li>举例：</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">ALTER TABLE	dept80</span><br><span class="line">MODIFY last_name VARCHAR(30);</span><br></pre></td></tr></table></figure>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">ALTER TABLE	dept80</span><br><span class="line">MODIFY salary double(9,2) default 1000;</span><br></pre></td></tr></table></figure>
<ul>
<li>对默认值的修改只影响今后对表的修改</li>
<li>此外，还可以通过此种方式修改列的约束。这里暂先不讲。</li>
</ul>
<h3 id="4-3-重命名一个列"><a href="#4-3-重命名一个列" class="headerlink" title="4.3 重命名一个列"></a>4.3 重命名一个列</h3><p>使用 CHANGE old_column  new_column  dataType子句重命名列。语法格式如下：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">ALTER TABLE 表名 CHANGE 【column】 列名 新列名 新数据类型;</span><br></pre></td></tr></table></figure>
<p>举例：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">ALTER TABLE  dept80</span><br><span class="line">CHANGE department_name dept_name varchar(15); </span><br></pre></td></tr></table></figure>
<h3 id="4-4-删除一个列"><a href="#4-4-删除一个列" class="headerlink" title="4.4 删除一个列"></a>4.4 删除一个列</h3><p>删除表中某个字段的语法格式如下：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">ALTER TABLE 表名 DROP 【COLUMN】字段名</span><br></pre></td></tr></table></figure>
<p>举例：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">ALTER TABLE  dept80</span><br><span class="line">DROP COLUMN  job_id; </span><br></pre></td></tr></table></figure>
<h2 id="5-重命名表"><a href="#5-重命名表" class="headerlink" title="5. 重命名表"></a>5. 重命名表</h2><ul>
<li>方式一：使用RENAME</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">RENAME TABLE emp</span><br><span class="line">TO myemp;</span><br></pre></td></tr></table></figure>
<ul>
<li>方式二：</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">ALTER table dept</span><br><span class="line">RENAME [TO] detail_dept;  -- [TO]可以省略</span><br></pre></td></tr></table></figure>
<ul>
<li>必须是对象的拥有者</li>
</ul>
<h2 id="6-删除表"><a href="#6-删除表" class="headerlink" title="6. 删除表"></a>6. 删除表</h2><ul>
<li><p>在MySQL中，当一张数据表<code>没有与其他任何数据表形成关联关系</code>时，可以将当前数据表直接删除。</p>
</li>
<li><p>数据和结构都被删除</p>
</li>
<li>所有正在运行的相关事务被提交</li>
<li>所有相关索引被删除</li>
<li>语法格式：</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">DROP TABLE [IF EXISTS] 数据表1 [, 数据表2, …, 数据表n];</span><br></pre></td></tr></table></figure>
<p><code>IF EXISTS</code>的含义为：如果当前数据库中存在相应的数据表，则删除数据表；如果当前数据库中不存在相应的数据表，则忽略删除语句，不再执行删除数据表的操作。</p>
<ul>
<li>举例：</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">DROP TABLE dept80;</span><br></pre></td></tr></table></figure>
<ul>
<li>DROP TABLE 语句不能回滚</li>
</ul>
<h2 id="7-清空表"><a href="#7-清空表" class="headerlink" title="7. 清空表"></a>7. 清空表</h2><ul>
<li><p>TRUNCATE TABLE语句：</p>
<ul>
<li>删除表中所有的数据</li>
<li>释放表的存储空间</li>
</ul>
</li>
<li><p>举例：</p>
</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">TRUNCATE TABLE detail_dept;</span><br></pre></td></tr></table></figure>
<ul>
<li><p>TRUNCATE语句<strong>不能回滚</strong>，而使用 DELETE 语句删除数据，可以回滚</p>
</li>
<li><p>对比：</p>
</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">SET autocommit = FALSE;</span><br><span class="line">  </span><br><span class="line">DELETE FROM emp2; </span><br><span class="line">#TRUNCATE TABLE emp2;</span><br><span class="line">  </span><br><span class="line">SELECT * FROM emp2;</span><br><span class="line">  </span><br><span class="line">ROLLBACK;</span><br><span class="line">  </span><br><span class="line">SELECT * FROM emp2;</span><br></pre></td></tr></table></figure>
<blockquote>
<p>阿里开发规范：</p>
<p>【参考】TRUNCATE TABLE 比 DELETE 速度快，且使用的系统和事务日志资源少，但 TRUNCATE 无事务且不触发 TRIGGER，有可能造成事故，故不建议在开发代码中使用此语句。 </p>
<p>说明：TRUNCATE TABLE 在功能上与不带 WHERE 子句的 DELETE 语句相同。</p>
</blockquote>
<h2 id="8-内容拓展"><a href="#8-内容拓展" class="headerlink" title="8. 内容拓展"></a>8. 内容拓展</h2><h3 id="拓展1：阿里巴巴《Java开发手册》之MySQL字段命名"><a href="#拓展1：阿里巴巴《Java开发手册》之MySQL字段命名" class="headerlink" title="拓展1：阿里巴巴《Java开发手册》之MySQL字段命名"></a>拓展1：阿里巴巴《Java开发手册》之MySQL字段命名</h3><ul>
<li><p>【<code>强制</code>】表名、字段名必须使用小写字母或数字，禁止出现数字开头，禁止两个下划线中间只出现数字。数据库字段名的修改代价很大，因为无法进行预发布，所以字段名称需要慎重考虑。</p>
<ul>
<li>正例：aliyun_admin，rdc_config，level3_name </li>
<li>反例：AliyunAdmin，rdcConfig，level_3_name</li>
</ul>
</li>
<li><p>【<code>强制</code>】禁用保留字，如 desc、range、match、delayed 等，请参考 MySQL 官方保留字。</p>
</li>
<li><p>【<code>强制</code>】表必备三字段：id, gmt_create, gmt_modified。 </p>
<ul>
<li>说明：其中 id 必为主键，类型为BIGINT UNSIGNED、单表时自增、步长为 1。gmt_create, gmt_modified 的类型均为 DATETIME 类型，前者现在时表示主动式创建，后者过去分词表示被动式更新</li>
</ul>
</li>
<li><p>【<code>推荐</code>】表的命名最好是遵循 “业务名称_表的作用”。 </p>
<ul>
<li>正例：alipay_task 、 force_project、 trade_config</li>
</ul>
</li>
<li><p>【<code>推荐</code>】库名与应用名称尽量一致。</p>
</li>
<li><p>【参考】合适的字符存储长度，不但节约数据库表空间、节约索引存储，更重要的是提升检索速度。 </p>
<ul>
<li>正例：无符号值可以避免误存负数，且扩大了表示范围。</li>
</ul>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/image-20211024012735469.webp" alt="image-20211024012735469"></p>
</li>
</ul>
<h3 id="拓展2：如何理解清空表、删除表等操作需谨慎？！"><a href="#拓展2：如何理解清空表、删除表等操作需谨慎？！" class="headerlink" title="拓展2：如何理解清空表、删除表等操作需谨慎？！"></a>拓展2：如何理解清空表、删除表等操作需谨慎？！</h3><p><code>表删除</code>操作将把表的定义和表中的数据一起删除，并且MySQL在执行删除操作时，不会有任何的确认信息提示，因此执行删除操时应当慎重。在删除表前，最好对表中的数据进行<code>备份</code>，这样当操作失误时可以对数据进行恢复，以免造成无法挽回的后果。</p>
<p>同样的，在使用 <code>ALTER TABLE</code> 进行表的基本修改操作时，在执行操作过程之前，也应该确保对数据进行完整的<code>备份</code>，因为数据库的改变是<code>无法撤销</code>的，如果添加了一个不需要的字段，可以将其删除；相同的，如果删除了一个需要的列，该列下面的所有数据都将会丢失。</p>
<h3 id="拓展3：MySQL8新特性—DDL的原子化"><a href="#拓展3：MySQL8新特性—DDL的原子化" class="headerlink" title="拓展3：MySQL8新特性—DDL的原子化"></a>拓展3：MySQL8新特性—DDL的原子化</h3><p>在MySQL 8.0版本中，InnoDB表的DDL支持事务完整性，即<code>DDL操作要么成功要么回滚</code>。DDL操作回滚日志写入到data dictionary数据字典表mysql.innodb_ddl_log（该表是隐藏的表，通过show tables无法看到）中，用于回滚操作。通过设置参数，可将DDL操作日志打印输出到MySQL错误日志中。</p>
<p>分别在MySQL 5.7版本和MySQL 8.0版本中创建数据库和数据表，结果如下：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">CREATE DATABASE mytest;</span><br><span class="line"></span><br><span class="line">USE mytest;</span><br><span class="line"></span><br><span class="line">CREATE TABLE book1(</span><br><span class="line">book_id INT ,</span><br><span class="line">book_name VARCHAR(255)</span><br><span class="line">);</span><br><span class="line"></span><br><span class="line">SHOW TABLES;</span><br></pre></td></tr></table></figure>
<p>（1）在MySQL 5.7版本中，测试步骤如下：<br>删除数据表book1和数据表book2，结果如下：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">mysql&gt; DROP TABLE book1,book2;</span><br><span class="line">ERROR 1051 (42S02): Unknown table &#x27;mytest.book2&#x27;</span><br></pre></td></tr></table></figure>
<p>再次查询数据库中的数据表名称，结果如下：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">mysql&gt; SHOW TABLES;</span><br><span class="line">Empty set (0.00 sec)</span><br></pre></td></tr></table></figure>
<p>从结果可以看出，虽然删除操作时报错了，但是仍然删除了数据表book1。</p>
<p>（2）在MySQL 8.0版本中，测试步骤如下：<br>删除数据表book1和数据表book2，结果如下：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">mysql&gt; DROP TABLE book1,book2;</span><br><span class="line">ERROR 1051 (42S02): Unknown table &#x27;mytest.book2&#x27;</span><br></pre></td></tr></table></figure>
<p>再次查询数据库中的数据表名称，结果如下：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">mysql&gt; show tables;</span><br><span class="line">+------------------+</span><br><span class="line">| Tables_in_mytest |</span><br><span class="line">+------------------+</span><br><span class="line">| book1            |</span><br><span class="line">+------------------+</span><br><span class="line">1 row in set (0.00 sec)</span><br></pre></td></tr></table></figure>
<p>从结果可以看出，数据表book1并没有被删除。</p>
</article><div class="post-copyright"><div class="post-copyright__title"><span class="post-copyright-info"><h>第十-MySQL创建和管理表</h></span></div><div class="post-copyright__type"><span class="post-copyright-info"><a href="https://www.naste.top:1024/posts/1882192809.html">https://www.naste.top:1024/posts/1882192809.html</a></span></div><div class="post-copyright-m"><div class="post-copyright-m-info"><div class="post-copyright-a"><h>作者</h><div class="post-copyright-cc-info"><h>XiaoFei🥝</h></div></div><div class="post-copyright-c"><h>发布于</h><div class="post-copyright-cc-info"><h>2022-09-19</h></div></div><div class="post-copyright-u"><h>更新于</h><div class="post-copyright-cc-info"><h>2024-04-26</h></div></div><div class="post-copyright-c"><h>许可协议</h><div class="post-copyright-cc-info"><a class="icon" rel="noopener" target="_blank" title="Creative Commons" href="https://creativecommons.org/"><i class="fab fa-creative-commons"></i></a><a rel="noopener" target="_blank" title="CC BY-NC-SA 4.0" 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class="toc-item toc-level-1"><a class="toc-link" href="#%E7%AC%AC10%E7%AB%A0-%E5%88%9B%E5%BB%BA%E5%92%8C%E7%AE%A1%E7%90%86%E8%A1%A8"><span class="toc-text">第10章_创建和管理表</span></a><ol class="toc-child"><li class="toc-item toc-level-2"><a class="toc-link" href="#1-%E5%9F%BA%E7%A1%80%E7%9F%A5%E8%AF%86"><span class="toc-text">1. 基础知识</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#1-1-%E4%B8%80%E6%9D%A1%E6%95%B0%E6%8D%AE%E5%AD%98%E5%82%A8%E7%9A%84%E8%BF%87%E7%A8%8B"><span class="toc-text">1.1 一条数据存储的过程</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#1-2-%E6%A0%87%E8%AF%86%E7%AC%A6%E5%91%BD%E5%90%8D%E8%A7%84%E5%88%99"><span class="toc-text">1.2 标识符命名规则</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#1-3-MySQL%E4%B8%AD%E7%9A%84%E6%95%B0%E6%8D%AE%E7%B1%BB%E5%9E%8B"><span class="toc-text">1.3 MySQL中的数据类型</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#2-%E5%88%9B%E5%BB%BA%E5%92%8C%E7%AE%A1%E7%90%86%E6%95%B0%E6%8D%AE%E5%BA%93"><span class="toc-text">2. 创建和管理数据库</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#2-1-%E5%88%9B%E5%BB%BA%E6%95%B0%E6%8D%AE%E5%BA%93"><span class="toc-text">2.1 创建数据库</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#2-2-%E4%BD%BF%E7%94%A8%E6%95%B0%E6%8D%AE%E5%BA%93"><span class="toc-text">2.2 使用数据库</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#2-3-%E4%BF%AE%E6%94%B9%E6%95%B0%E6%8D%AE%E5%BA%93"><span class="toc-text">2.3 修改数据库</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#2-4-%E5%88%A0%E9%99%A4%E6%95%B0%E6%8D%AE%E5%BA%93"><span class="toc-text">2.4 删除数据库</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#3-%E5%88%9B%E5%BB%BA%E8%A1%A8"><span class="toc-text">3. 创建表</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a 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href="#4-3-%E9%87%8D%E5%91%BD%E5%90%8D%E4%B8%80%E4%B8%AA%E5%88%97"><span class="toc-text">4.3 重命名一个列</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#4-4-%E5%88%A0%E9%99%A4%E4%B8%80%E4%B8%AA%E5%88%97"><span class="toc-text">4.4 删除一个列</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#5-%E9%87%8D%E5%91%BD%E5%90%8D%E8%A1%A8"><span class="toc-text">5. 重命名表</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#6-%E5%88%A0%E9%99%A4%E8%A1%A8"><span class="toc-text">6. 删除表</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#7-%E6%B8%85%E7%A9%BA%E8%A1%A8"><span class="toc-text">7. 清空表</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#8-%E5%86%85%E5%AE%B9%E6%8B%93%E5%B1%95"><span class="toc-text">8. 内容拓展</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" 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    arr[i].setAttribute('data-wow-delay', '200ms');
    arr[i].setAttribute('data-wow-offset', '30');
    arr[i].setAttribute('data-wow-iteration', '1');
  }</script><script async="async">var arr = document.getElementsByClassName('card-widget');
for(var i = 0;i<arr.length;i++){
    arr[i].classList.add('wow');
    arr[i].classList.add('animate__zoomIn');
    arr[i].setAttribute('data-wow-duration', '2s');
    arr[i].setAttribute('data-wow-delay', '200ms');
    arr[i].setAttribute('data-wow-offset', '30');
    arr[i].setAttribute('data-wow-iteration', '1');
  }</script></div><script defer src="https://npm.elemecdn.com/hexo-butterfly-wowjs/lib/wow.min.js"></script><script defer src="https://npm.elemecdn.com/hexo-butterfly-wowjs/lib/wow_init.js"></script><script data-pjax src="https://npm.elemecdn.com/hexo-filter-gitcalendar/lib/gitcalendar.js"></script><script data-pjax>
  function gitcalendar_injector_config(){
      var parent_div_git = document.getElementById('gitZone');
      var item_html = '<div class="recent-post-item" id="gitcalendarBar" style="width:100%;height:auto;padding:10px;"><style>#git_container{min-height: 320px}@media screen and (max-width:650px) {#git_container{min-height: 0px}}</style><div id="git_loading" style="width:10%;height:100%;margin:0 auto;display: block;"><svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" viewBox="0 0 50 50" style="enable-background:new 0 0 50 50" xml:space="preserve"><path fill="#d0d0d0" d="M25.251,6.461c-10.318,0-18.683,8.365-18.683,18.683h4.068c0-8.071,6.543-14.615,14.615-14.615V6.461z" transform="rotate(275.098 25 25)"><animatetransform attributeType="xml" attributeName="transform" type="rotate" from="0 25 25" to="360 25 25" dur="0.6s" repeatCount="indefinite"></animatetransform></path></svg><style>#git_container{display: none;}</style></div><div id="git_container"></div></div>';
      parent_div_git.insertAdjacentHTML("afterbegin",item_html)
      console.log('已挂载gitcalendar')
      }

    if( document.getElementById('gitZone') && (location.pathname ==='/personal/about/'|| '/personal/about/' ==='all')){
        gitcalendar_injector_config()
        GitCalendarInit("https://gitcalendar.naste.top/api?xiao-i-fei",['#d9e0df', '#c6e0dc', '#a8dcd4', '#9adcd2', '#89ded1', '#77e0d0', '#5fdecb', '#47dcc6', '#39dcc3', '#1fdabe', '#00dab9'],'xiao-i-fei')
    }
  </script><!-- hexo injector body_end end --></body></html>